Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density, since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an aluminum electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them, all placed within the capacitor case.
Currently, etched/formed aluminum anode foil is punched by use of a mechanical die into an anode shape to conform to the necessary geometry of the capacitor case. In order to obtain higher capacitance, aluminum is removed from the anode foil during an etching process to create tunnels to increase surface area. A widening process is then used to open the tunnels to prevent clogging during a later oxide formation step. Both the etch and widening processes can remove as much as 50% to 60% of the aluminum to create greater than 30 million tunnels per cm2. After the formation of the oxide, the foil becomes very brittle. The more aluminum removed (higher surface area), the harder the foil is to punch without creating cracks and particles.
After the anodes are punched by the mechanical die, the anodes are assembled into stacks with the paper and the cathode(s). The edges of the anodes can contain burrs and attached particles. The burrs and particles can penetrate the paper layer and cause a short circuit, which could compromise the quality and life of the capacitor.
Additionally, after the punching process, the newly created edges of the anodes have exposed aluminum without a high quality oxide formed thereon. After the assembly of the anodes, paper, and cathodes, the capacitor is sealed and impregnated with an electrolyte. Next, the capacitors are put through an aging process that forms oxide on the edges and any exposed cracks in the anodes, but the oxide formed from the aging process is not as high quality as the oxide formed during the formation process. The higher the edge area to anode surface area, the higher the leakage current of the capacitor. Additionally, the higher the edge area to anode surface area, the longer the aging process is required to be performed to form oxide on the edges.